The present invention relates to crucibles for the fusion of samples for analysis and particularly to crucibles made of a resistive material.
In resistance furnaces for fusing specimens to obtain gas samples therefrom of the type manufactured by Leco Corporation of St. Joseph, Mich., as an integral part of an analyzer such as a model TC-36, graphite sample-holding crucibles are employed for containing the sample. The crucible is typically positioned between actuating electrodes for applying pulsed current thereto for heating the sample to a fusion temperature in excess of 3000.degree. C. Several crucible designs have previously been employed in such furnaces including crucibles of the type described in U.S. Pat. Nos. 3,636,229, and 3,899,627, both of which are assigned to the assignee of the present invention.
It has been discovered that during fusion of samples in such crucibles, the typically metallic sample tends to diffuse into the graphite sidewalls of the crucible thereby changing its resistive characteristics and accordingly, the temperature at which a crucible will heat a sample as the operating power is applied to it. Although this is not a serious problem when the crucibles are employed for rapidly fusing specimens at a relatively high rate, it adversely affects the temperature characteristics if it is desired to uniformly and gradually heat the crucible in a predictable pattern. Thus, for example, in an analyzer of the type described in co-pending U.S. application Ser. No. 190,357 Method and Apparatus for Controlling an Analytical Furnace filed concurrently herewith, it is desired to increase the temperature of the fusion furnace in a stepwise fashion which is directly related to the applied power. To achieve this, based upon application of power alone, it is necessary to assure that the graphite crucible maintain a substantially constant resistance as increasing power is applied to achieve the predictable temperature increases.
Multiple piece crucible assemblies have been provided in the prior art. For example, one such crucible design as shown in FIG. 1 of the present application in which two cylindrically shaped crucibles are employed with one telescopically fitting within the other. Such crucible is specifically designed for an analyzer which provides rapid fusion of the sample within the inner crucible with the outer crucible providing the heat of fusion necessary to combust the sample. The specific structure of this crucible however does not render it operationally advantageous for use in the environment of the present invention.